U.S. patent application number 10/631977 was filed with the patent office on 2005-02-03 for recuperated gas turbine engine system and method employing catalytic combustion.
This patent application is currently assigned to MES International, Inc.. Invention is credited to Belokon, Alexander A., Touchton, George L..
Application Number | 20050022499 10/631977 |
Document ID | / |
Family ID | 34104237 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050022499 |
Kind Code |
A1 |
Belokon, Alexander A. ; et
al. |
February 3, 2005 |
Recuperated gas turbine engine system and method employing
catalytic combustion
Abstract
A recuperated gas turbine engine system and associated method
employing catalytic combustion, wherein the combustor inlet
temperature can be controlled to remain above the minimum required
catalyst operating temperature at a wide range of operating
conditions from full-load to part-load and from hot-day to cold-day
conditions. The fuel is passed through the compressor along with
the air and a portion of the exhaust gases from the turbine. The
recirculated exhaust gas flow rate is controlled to control
combustor inlet temperature.
Inventors: |
Belokon, Alexander A.;
(Moscow, RU) ; Touchton, George L.; (Newark,
CA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
MES International, Inc.
|
Family ID: |
34104237 |
Appl. No.: |
10/631977 |
Filed: |
July 31, 2003 |
Current U.S.
Class: |
60/39.511 |
Current CPC
Class: |
F23N 2237/12 20200101;
F23C 2202/10 20130101; F23N 2241/20 20200101; F23C 9/00 20130101;
F23R 3/40 20130101 |
Class at
Publication: |
060/039.511 |
International
Class: |
F02C 007/10 |
Claims
What is claimed is:
1. A recuperated gas turbine engine system employing catalytic
combustion, comprising: a compressor arranged to receive air and to
compress the air; a fuel system operable to supply fuel into the
compressor, such that a mixture of compressed air and fuel is
discharged from the compressor; a catalytic combustor operable to
combust the mixture to produce hot combustion gases; a turbine
arranged to receive the combustion gases and expand the gases to
produce mechanical power that drives the compressor; a recuperator
arranged to receive exhaust gases from the turbine and the mixture
discharged from the compressor and cause heat exchange therebetween
such that the mixture is pre-heated before entering the catalytic
combustor; and a system operable to direct a portion of turbine
exhaust gases into the compressor, such that the mixture discharged
from the compressor is raised in temperature by said exhaust gases,
whereby an inlet temperature to the catalytic combustor is
raised.
2. The recuperated gas turbine engine system of claim 1, wherein
the system operable to direct a portion of turbine exhaust gases
into the compressor includes a valve that is controllable to
variably adjust a flow rate of the exhaust gases into the
compressor, and a control system operably connected to the
valve.
3. The recuperated gas turbine engine system of claim 2, wherein
the control system includes a sensor operable to measure a
parameter indicative of combustor inlet temperature, the control
system being operable to control the valve in a manner to cause the
combustor inlet temperature to exceed a predetermined minimum
temperature necessary for proper operation of the catalytic
combustor.
4. The recuperated gas turbine engine system of claim 3, wherein
the control system further comprises a sensor operable to measure
air flow rate and a sensor operable to measure fuel flow rate, and
a sensor operable to measure recuperator inlet temperature, the
control system operable to determine fuel/air ratio of the mixture
entering the combustor based on the flow rates of air, fuel, and
exhaust gases, and to control the flow rate of exhaust gases into
the compressor so as to optimize the combustor inlet temperature
for said fuel/air ratio in such a manner that a maximum allowable
recuperator temperature is not exceeded.
5. The recuperated gas turbine engine system of claim 4, wherein
the control system is further operable to control the combustor
inlet temperature for said fuel/air ratio in such a manner that an
efficiency of the engine is maximized.
6. The recuperated gas turbine engine system of claim 5, further
comprising means for determining a level of emissions from the
engine, and wherein the control system is operable to control the
combustor inlet temperature for said fuel/air ratio in such a
manner that a maximum allowable emissions limit is not
exceeded.
7. The recuperated gas turbine engine system of claim 6, wherein
the means for determining a level of emissions comprises an
emissions sensor.
8. The recuperated gas turbine engine system of claim 5, further
comprising means for determining a level of emissions from the
engine, and wherein the control system is operable to control the
combustor inlet temperature for said fuel/air ratio in such a
manner that emissions are minimized.
9. The recuperated gas turbine engine system of claim 2, wherein
the valve is located downstream of the recuperator such that the
exhaust gases are cooled in the recuperator before being directed
into the compressor.
10. The recuperated gas turbine engine system of claim 2, wherein
the valve is located upstream of the recuperator such that the
portion of exhaust gas bypasses the recuperator and is then
directed into the compressor.
11. The recuperated gas turbine engine system of claim 1, further
comprising an electrical generator arranged to be driven by the
turbine.
12. A method for operating a gas turbine engine, comprising the
steps of: compressing air in a compressor; mixing fuel with
compressed air from the compressor to produce an air-fuel mixture;
burning the air-fuel mixture in a catalytic combustor to produce
hot combustion gases; expanding the combustion gases in a turbine
to produce mechanical power, and using the mechanical power to
drive the compressor; passing exhaust gases from the turbine
through a recuperator and passing the air-fuel mixture through the
recuperator to pre-heat the mixture by heat exchange with the
exhaust gases; directing a portion of exhaust gases from the
turbine into the compressor to raise an inlet temperature to the
combustor; and wherein the fuel is passed through the compressor
along with the air and the portion of exhaust gases.
13. The method of claim 12, wherein mixing of the exhaust gases
with the fuel is accomplished upstream of the compressor.
14. The method of claim 13, wherein the mixed exhaust gases and
fuel are directed into the compressor separately from the air.
15. The method of claim 12, wherein at least some mixing of the
fuel with the air is accomplished upstream of the compressor.
16. The method of claim 15, wherein the mixed fuel and air are
directed into the compressor separately from the exhaust gases.
17. The method of claim 12, wherein the air, fuel, and exhaust
gases are directed into the compressor separately from one another
and mixing takes place in the compressor.
18. The method of claim 12, further comprising the step of
controlling a flow rate of the exhaust gases directed into the
compressor.
19. The method of claim 18, wherein the controlling step comprises
controlling the flow rate in response to a parameter associated
with the engine.
20. The method of claim 19, wherein the controlling step comprises
controlling the flow rate in response to a measured combustor inlet
temperature.
21. The method of claim 20, wherein the flow rate is controlled so
as to always maintain the combustor inlet temperature higher than a
predetermined minimum temperature necessary for proper operation of
the catalytic combustor.
22. The method of claim 21, further comprising the step of deducing
fuel/air ratio of the mixture entering the combustor, and
controlling the combustor inlet temperature so as to optimize the
combustor inlet temperature for said fuel/air ratio in such a
manner that at all times a maximum allowable recuperator
temperature is not exceeded.
23. The method of claim 21, further comprising the step of deducing
fuel/air ratio of the mixture entering the combustor, and
controlling the combustor inlet temperature so as to optimize the
combustor inlet temperature for said fuel/air ratio in such a
manner that a maximum allowable emissions limit is not
exceeded.
24. The method of claim 23, further comprising the step of deducing
fuel/air ratio of the mixture entering the combustor, and
controlling the combustor inlet temperature so as to optimize the
combustor inlet temperature for said fuel/air ratio in such a
manner that an efficiency of the engine is maximized.
25. The method of claim 21, further comprising the step of deducing
fuel/air ratio of the mixture entering the combustor, and
controlling the combustor inlet temperature so as to optimize the
combustor inlet temperature for said fuel/air ratio in such a
manner that emissions are minimized.
26. The method of claim 25, further comprising the step of deducing
fuel/air ratio of the mixture entering the combustor, and
controlling the combustor inlet temperature so as to optimize the
combustor inlet temperature for said fuel/air ratio in such a
manner that efficiency is maximized.
27. The method of claim 19, wherein the controlling step comprises
controlling the flow rate to compensate for changes in ambient
temperature.
28. The method of claim 27, wherein a relative portion of the
exhaust gases directed into the compressor is increased when there
is a decrease in ambient temperature.
29. The method of claim 19, wherein the controlling step comprises
controlling the flow rate to compensate for changes in relative
engine load.
30. The method of claim 29, wherein a relative proportion of the
exhaust gases directed into the compressor is increased when there
is a decrease in relative engine load.
31. The method of claim 12, wherein the portion of exhaust gases
directed into the compressor is separated from the remainder of the
exhaust gases at a point downstream of the recuperator.
32. The method of claim 12, wherein the portion of exhaust gases
directed into the compressor is separated from the remainder of the
exhaust gases at a point upstream of the recuperator such that said
portion bypasses the recuperator.
33. The method of claim 12, further comprising the step of driving
an electrical generator with the turbine.
Description
FIELD OF THE INVENTION
[0001] The invention relates to recuperated gas turbine engine
systems in which catalytic combustion is employed.
BACKGROUND OF THE INVENTION
[0002] The use of catalytic processes for combustion or oxidation
is a well-known method for potentially reducing levels of nitrogen
oxides (NO.sub.x) emissions from gas turbine engine systems. There
are various processes for converting the chemical energy in a fuel
to heat energy in the products of the conversion. The primary
processes are: 1) gas phase combustion, 2) catalytic combustion,
and 3) catalytic oxidation. There are also combinations of these
processes, such as processes having a first stage of catalytic
oxidation followed by a gas phase combustion process (often
referred to as cata-thermal). In catalytic oxidation, an air-fuel
mixture is oxidized in the presence of a catalyst. In all catalytic
processes the catalyst allows the temperature at which oxidation
takes place to be reduced relative to non-catalytic combustion
temperatures. Lower oxidation temperature leads to reduced NO.sub.x
production. In catalytic oxidation all reactions take place on the
catalytic surface; there are no local high temperatures and
therefore the lowest possible potential for NO.sub.x to be formed.
In either catalytic combustion or catathermal combustion, some part
of the reaction takes place in the gas phase, which increases local
temperatures and leads to higher potential for NO.sub.x being
formed. Using catalytic oxidation, NO.sub.x levels less than one
part per million can be achieved under optimum catalytic oxidation
conditions; such low levels in general cannot be achieved with
conventional non-catalytic combustors, catalytic combustion, or
cata-thermal combustion. In the present application, the term
"catalytic combustor" is used to refer to any combustor utilizing
catalysis, preferably one utilizing catalytic oxidation.
[0003] The catalyst employed in a catalytic combustor tends to
operate best under certain temperature conditions. In particular,
there is typically a minimum temperature below which a given
catalyst will not function. For instance, palladium catalyst
requires a combustor inlet temperature for the air-fuel mixture
higher than 800 K when natural gas is the fuel. In addition,
catalytic oxidation has the disadvantage that the physical reaction
surface which must be supplied for complete oxidation of the
hydrocarbon fuel increases exponentially with decreasing combustor
inlet temperatures, which greatly increases the cost of the
combustor and complicates the overall design. The need for a
relatively high combustor inlet temperature is one of the chief
reasons why catalytic combustion in general, and catalytic
oxidation in particular, has not achieved widespread use in gas
turbine engine systems. More specifically, such high combustor
inlet temperatures generally cannot be achieved in gas turbines
operating with compressor pressure ratios less than about 40 unless
a recuperated cycle is employed. In a recuperated cycle, the
air-fuel mixture is pre-heated, prior to combustion, by heat
exchange with the turbine exhaust gases. Recuperation thus can help
achieve the needed combustor inlet temperature for proper catalyst
operation, at least under some conditions. However, there are often
other operating conditions that will be encountered at which the
minimum required combustor inlet temperature still cannot be
achieved even with recuperation.
[0004] For instance, when recuperation is applied in small gas
turbines, material temperature limitations in the recuperator can
limit the maximum air or air-fuel mixture temperature. As an
example, with conventional high-temperature materials in the
recuperator, the maximum safe operating temperature of the
recuperator may be about 900 K, and hence an air-fuel mixture
temperature of about 800 to 850 K is about the highest that can be
achieved. This temperature range is higher than the minimum
catalyst operating temperature for some types of catalysts and
therefore the catalytic combustor may operate properly at one
particular operating condition such as 100 percent load and
standard-day ambient conditions. At other operating conditions,
however, such as part-load and/or cold ambient conditions, the
combustor inlet temperature may fall below the minimum.
[0005] It would be desirable to be able to overcome such problems
so that the low-NO.sub.x potential of catalytic oxidation could be
realized in small gas turbine engine systems. Additionally, there
are other benefits that can be achieved with catalytic processes.
These processes extend the operating flammability limits of gaseous
hydrocarbon fuels, including but not limited to landfill gases,
anaerobic digester gases, natural gas, and methane. Thus, the
process can take place at much more dilute (leaner) fuel/air ratios
than conventional combustion. This allows the fuel gas to be mixed
with the air prior to or during the compression process, resulting
in a uniform fuel-air mixture entering the combustor. This in turn
allows the elimination of a fuel gas compressor, which is very
costly particularly for small gas turbines. Fuel gas compressors
may add $60/kW or more to the cost of the engine, which is
typically in the range of $600-$900/kW. Furthermore, the fuel gas
compressor detracts from the reliability and availability of the
engine, since it must operate in order for the engine to operate,
and adds to the cost of maintenance because of oil, filters,
mechanical or electrical wear out, and the like.
SUMMARY OF THE INVENTION
[0006] The present invention addresses the above needs and achieves
other advantages, by providing a recuperated gas turbine engine
system and associated method employing catalytic oxidation or
combustion or cata-thermal combustion, wherein the combustor inlet
temperature can be controlled to remain above the minimum required
catalyst operating temperature, and further optimized as a function
of fuel/air ratio, at a wide range of operating conditions from
full-load to part-load and from hot-day to cold-day conditions.
[0007] In accordance with a method aspect of the invention, a
method for operating a gas turbine engine comprises steps of
compressing air in a compressor, mixing fuel with compressed air
from the compressor to produce an air-fuel mixture, burning the
air-fuel mixture in a catalytic combustor to produce hot combustion
gases, expanding the combustion gases in a turbine to produce
mechanical power and using the mechanical power to drive the
compressor, and passing exhaust gases from the turbine through a
recuperator in which the air-fuel mixture is pre-heated by heat
exchange with the exhaust gases. The method includes the further
step of directing a portion of exhaust gases from the turbine into
the compressor. The fuel is also passed through the compressor
along with the air and the portion of exhaust gases. The
recirculation of the exhaust gas raises the inlet temperature to
the combustor above what it would be without the exhaust gas
recirculation. Ultimately what enters the combustor is a mixture of
the air, fuel, and exhaust gases optimized to meet power output,
maximize efficiency, and minimize air pollution
[0008] The mixing of the air, fuel, and exhaust gases can be
accomplished in various ways. In one embodiment, mixing of the
exhaust gases with the fuel is accomplished upstream of the
compressor, and the mixed exhaust gases and fuel are directed into
the compressor separately from the air. Alternatively, at least
some mixing of the fuel with the air can be accomplished upstream
of the compressor, and the mixed fuel and air can be directed into
the compressor separately from the exhaust gases. As yet another
alternative, the air, fuel, and exhaust gases are directed into the
compressor separately from one another and mixing takes place in
the compressor or passages associated with the compressor and other
components.
[0009] In accordance with the invention, the flow rate of the
exhaust gases directed into the compressor is controlled in
response to one or more parameters associated with the engine, at
least one of which is the fuel/air ratio. For instance, the
controlling step can comprise controlling the flow rate in response
to a measured combustor inlet temperature so as to maintain the
combustor inlet temperature higher than a predetermined minimum
temperature necessary for proper operation of the catalytic
combustor at that fuel/air ratio. In this manner, the flow rate of
the exhausts gases into the compressor can be optimized to
compensate for changes in ambient temperature and/or relative
engine load.
[0010] The portion of exhaust gases directed into the compressor
can be separated from the remainder of the exhaust gases at a point
downstream of the recuperator. In this case, the recirculated
exhaust gases will be reduced in temperature by their passage
through the recuperator. Alternatively, the portion of exhaust
gases directed into the compressor can be separated from the
remainder of the exhaust gases at a point upstream of the
recuperator such that the recirculated exhaust gases bypass the
recuperator. In such an arrangement, the temperature of the
recirculated exhaust gases fed to the compressor will be higher and
therefore the recirculated exhaust gas flow rate can be lower than
in the previously described arrangement.
[0011] A recuperated gas turbine engine system employing catalytic
combustion in accordance with the invention comprises a compressor
arranged to receive air and to compress the air, a fuel system
operable to supply fuel into the compressor such that a mixture of
compressed air and fuel is discharged from the compressor, a
catalytic combustor operable to combust the mixture to produce hot
combustion gases, a turbine arranged to receive the combustion
gases and expand the gases to produce mechanical power that drives
the compressor, a recuperator arranged to receive exhaust gases
from the turbine and the mixture discharged from the compressor and
cause heat exchange therebetween such that the mixture is
pre-heated before entering the catalytic combustor, and a
recirculation system operable to direct a portion of turbine
exhaust gases into the compressor, such that the mixture discharged
from the compressor is raised in temperature by the exhaust gases,
whereby an inlet temperature to the catalytic combustor is
raised.
[0012] The recirculation system can include a valve that is
controllable to variably adjust a flow rate of the exhaust gases
into the compressor, and a control system operably connected to the
valve. Sensors operable to measure parameters indicative of
fuel/air ratio and combustor inlet temperature can be connected to
the control system, and the control system can be operable to
control the valve in a manner to cause the combustor inlet
temperature to exceed a predetermined minimum temperature necessary
for proper operation of the catalytic combustor and to match an
optimal temperature for the measured fuel/air ratio. As noted, the
valve can be upstream or downstream of the recuperator.
[0013] The recuperated engine system in accordance with the
invention has utility in various applications, including small
electrical power generation systems. Thus, an electrical generator
can be arranged to be driven by the turbine.
[0014] The system is not limited to single-spool turbine engines,
but can also be applied to multiple-spool engines or ganged systems
of single-spool engines.
[0015] The benefits of the present system and method will be
greatest for catalytic oxidation processes, but all processes
employing catalysis will benefit.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0016] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0017] FIG. 1 is a diagrammatic depiction of a turbine engine
system in accordance with the prior art;
[0018] FIG. 2 is a diagrammatic depiction of a turbine engine
system in accordance with a first embodiment of the invention;
[0019] FIG. 3 is a diagrammatic depiction of a turbine engine
system in accordance with a second embodiment of the invention;
[0020] FIG. 4 is a graph showing model calculations of turbine
inlet temperature, combustor inlet temperature, efficiency, and
compressor inlet temperature as a function of relative load, for
both a prior-art turbine engine system without exhaust gas mixing
at the compressor inlet, and a turbine engine system in accordance
with the invention having exhaust gas mixing at the compressor
inlet;
[0021] FIG. 5A depicts another embodiment of the invention in which
fuel and exhaust gas are mixed and fed into the compressor separate
from the air, such that mixing with air takes place entirely in the
compressor;
[0022] FIG. 5B shows a further embodiment in which the air and fuel
are mixed before being fed into the compressor, and the exhaust gas
is separately fed into the compressor; and
[0023] FIG. 5C shows yet another embodiment in which the air, fuel,
and exhaust gas are all separately fed into the compressor where
they are mixed.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present inventions now will be described more fully
hereinafter with reference to the accompanying drawings, in which
some, but not all embodiments of the invention are shown. Indeed,
these inventions may be embodied in many different forms and should
not be construed as limited to the embodiments set forth herein;
rather, these embodiments are provided so that this disclosure will
satisfy applicable legal requirements. Like numbers refer to like
elements throughout.
[0025] A prior-art electrical generation system 10 driven by a
recuperated gas turbine engine with catalytic combustion is shown
in FIG. 1. The system includes a gas turbine engine 12 comprising a
compressor 14 and a turbine 16 connected by a shaft 18 so as to
drive the compressor, and a catalytic combustor 20. The system also
includes a heat exchanger or recuperator 22 having one or more
passages 24 for compressor discharge fluid, arranged in
heat-transfer relationship with one or more passages 26 for turbine
exhaust gas. The system further includes an arrangement 28 for
bringing together and mixing air and fuel and feeding the mixture
into the compressor 14.
[0026] The compressed air-fuel mixture is pre-heated in the
recuperator 22 and is then fed into the catalytic combustor 20
where combustion takes place. The hot combustion gases are led from
the combustor into the turbine 16, which expands the hot gases to
produce mechanical power, which power is transmitted by the shaft
18 to the compressor 16. Also linked to the shaft is an electrical
generator 30, which is driven to produce electrical current for
supply to a load.
[0027] In a system such as shown in FIG. 1, it is possible to
design the engine components such that at relatively high engine
loads and standard-day conditions, the temperature of the air-fuel
mixture fed into the catalytic combustor 20 is at or above the
catalyst minimum temperature required for proper operation of the
catalytic reaction. The most widely used palladium catalyst
requires a combustor inlet temperature of at least 800 K. At low
loads and/or cold ambient conditions, however, the combustor inlet
temperature can fall below the catalyst minimum. See the dashed
lines in FIG. 4, representing model calculations of various
thermodynamic variables as a function of relative load, for the
prior-art type of cycle shown in FIG. 1. At a 100% load condition,
the combustor inlet temperature is about 850 K, but drops to the
catalyst minimum of 800 K at about 80% load. At still lower loads,
the combustor inlet temperature is too low to support proper
operation of the catalytic combustor.
[0028] The present invention provides a gas turbine engine system
and method that overcome this problem. FIG. 2 shows an electrical
generator system driven by a turbine engine system in accordance
with a first embodiment of the invention. A generator 30 is driven
by a turbine engine 12 having a compressor 14, turbine 16, shaft
18, and catalytic combustor 20 as previously described. A
recuperator 22 is employed for pre-heating the air-fuel mixture
before its introduction into the combustor, as previously
described.
[0029] However, the combustor inlet temperature is regulated by the
introduction of a portion of the turbine exhaust gas into the
compressor. The exhaust gas has a substantially higher temperature
than the ambient air entering the compressor, and therefore serves
to boost the temperature of the fluid passing through the
compressor, which in turn boosts the combustor inlet
temperature.
[0030] Thus, the system includes an actuatable valve 40 disposed
downstream of the recuperator 22 for diverting a portion of the
turbine exhaust gas through a line 42 to a mixer 44. The mixer 44
also receives at least two of air, fuel, and exhaust and mixes at
least two of the three constituents at least partially. The mixture
is then fed into the compressor 14, where further mixing may occur.
Any third unmixed stream may be introduced into the compressor
simultaneously with the other two and mixed therein or in
subsequent passages before reaching the recuperator.
[0031] The valve 40 is operable to selectively vary the amount of
turbine exhaust gas delivered through the line 42 to the mixer 44.
Additionally, the valve is controllable by a control system 50
(which may be a PC, a PLC, a neural network, or the like) that is
responsive to a temperature signal from a temperature sensor 52
arranged for detecting the combustor inlet temperature. The control
system can also be responsive to an airflow signal from an airflow
sensor 54 arranged for detecting the air flow rate, and a fuel flow
signal from a fuel flow sensor 56 arranged for detecting fuel flow
rate. Sensors 58 for detecting emissions, particularly unburned
hydrocarbons, can also be arranged in the exhaust duct after the
recuperator, if desired, and the measured emissions can be taken
into account by the control system. Alternatively, the emissions
may be calculated from the combustor inlet temperature and fuel/air
ratio using models determined from theory and engine testing.
Additionally, a sensor 60 for measuring recuperator inlet
temperature can also be employed. Although the connecting lines
between the sensors 54, 56, 58, and 60 and the control system 50
are not shown in FIGS. 2 and 3, it will be understood that these
sensors are connected to the control system. The control system is
suitably programmed to control the operation of the valve 40 so as
to regulate the combustor inlet temperature as desired. In
particular, the control system preferably includes logic for
open-loop or closed-loop control of the valve 40 in such a manner
that the combustor inlet temperature always equals or exceeds a
predetermined minimum temperature necessary for proper catalytic
reaction in the combustor. Advantageously, the control is also
carried out so that the recuperator inlet temperature does not
exceed the maximum allowable recuperator inlet temperature,
preferably while simultaneously minimizing emissions (or
maintaining them below desired limits) and maximizing efficiency.
Generally, as load drops, the proportion of turbine exhaust gas
that must be fed back into the compressor will increase so as to
maintain combustor inlet temperature above the predetermined
minimum level.
[0032] The effect of exhaust gas mixing with the air and fuel is
shown in solid lines on FIG. 4. As load drops, the compressor inlet
temperature increases, reflecting the greater and greater
proportion of exhaust gas being recirculated to the compressor. As
a result, the combustor inlet temperature is maintained above 800 K
for all load conditions. At the same time, in preferred
embodiments, the recuperator inlet temperature is prevented from
exceeding its maximum allowable value at all operating conditions,
and the efficiency of the engine is optimized, via simultaneous
control of the recirculated exhaust gas flow rate and fuel/air
ratio.
[0033] It will be appreciated that the same system and method can
compensate for changing ambient temperature. Thus, as ambient
temperature decreases, the proportion of recirculated exhaust gas
can be increased, if necessary, to maintain the needed combustor
inlet temperature. The combined effects of changing load and
ambient temperature can also be compensated for by the system and
method of the invention.
[0034] FIG. 3 shows a second embodiment of the invention, generally
similar to that of FIG. 2, except the valve 40 is located upstream
of the recuperator 22 instead of downstream. The line 42 thus
bypasses the recuperator, so the exhaust gas is not cooled in the
recuperator before being recirculated. Because the temperature of
the recirculated exhaust gas is higher, the relative proportion of
exhaust gas that must be recirculated is lower than for the
embodiment of FIG. 2, all other factors being equal. In other
respects, the operation of this system is the same as that of FIG.
2.
[0035] The manner in which the exhaust gas is recirculated and
mixed with the air and fuel can be varied in the practice of the
invention. FIGS. 5A-C show several possibilities, although they are
not exhaustive, and other variations can be used. All of these
examples are based on the valve 40 being downstream of the
recuperator 22, but they apply equally to systems in which the
valve is upstream of the recuperator. In the embodiment of FIG. 5A,
the recirculated exhaust gas is mixed with fuel in the mixer 44,
and the resulting mixture is fed into the compressor 14 separately
from the air. This arrangement may be advantageous when the fuel is
initially in liquid form (e.g., propane) in that the hot exhaust
gas will vaporize at least part of the fuel before it is fed into
the compressor.
[0036] In the arrangement of FIG. 5B, air and fuel are mixed in the
mixer 44 and the resulting mixture is fed into the compressor. The
exhaust gas from the line 42 is fed into the compressor separately,
and mixing with the air and fuel occurs in the compressor.
[0037] Yet another possibility is shown in FIG. 5C, where the air,
fuel, and exhaust gas are all fed separately into the compressor,
and mixing between all three occurs in the compressor.
[0038] Many modifications and other embodiments of the inventions
set forth herein will come to mind to one skilled in the art to
which these inventions pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the inventions are
not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims. Although specific terms
are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
* * * * *